Optical Coherence Tomography (OCT) is an interferometric technique for performing high-resolution cross-sectional imaging that can provide images of samples including tissue structure on the micron scale in situ and in real time. OCT is based on the principle of low coherence interferometry (LCI) and determines the scattering profile of a sample along the OCT beam by detecting the interference of light reflected or scattered from a sample and a reference beam. Each scattering profile in the depth direction (z) is called an axial scan, or A-scan. Cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse (x and y) locations on the sample. In time domain OCT (TD-OCT), the path length between light returning from the sample and reference light is translated longitudinally in time to recover the depth information in the sample. In frequency-domain or Fourier-domain OCT (FD-OCT), a method based on diffraction tomography, the broadband interference between reflected sample light and reference light is acquired in the spectral frequency domain and a Fourier transform is used to recover the depth information. The sensitivity advantage of FD-OCT over TD-OCT is well established.
There are two common approaches to FD-OCT. One is spectral domain OCT (SD-OCT) where the interfering light is spectrally dispersed prior to detection and the full depth information can be recovered from a single exposure. The second is swept-source OCT (SS-OCT) where the source is swept over a range of optical frequencies and detected in time, therefore encoding the spectral information in time. In traditional point scanning or flying spot techniques, a single point of light is scanned across the sample. The techniques have found great use in the field of ophthalmology.
Optical coherence tomography is able to observe extremely weak reflections in a sample and locate those reflections with high accuracy. When an OCT system is functioning optimally it is described as functioning close to the shot noise limit, indicating that signal-to-noise ratio is absolutely limited by the number of photons exiting the sample and being collected by its detectors. The power with which a sample such as the eye can be illuminated is limited potentially by safety and by the availability of source power available at a particular cost. Therefore, it has been a goal of OCT designers since the beginning of the field to develop interferometers and optical components which optimize the efficiency of light transfer, most critically in the direction from the sample to detector, but also with regard to the total amount of light produced by the source.
In order to achieve shot noise limited detection, the amount of power in the reference arm must be optimized. If the power in the reference is too low, detector noise dominates. If the power in the reference arm is too high, relative intensity noise from the source may dominate. Mechanical and optical tolerances and design uncertainty are frequently larger than the optimum reference arm power window, therefore adjustable elements are frequently built into OCT systems to allow compensation. A stable, low cost, and high efficiency means to adjustably set the reference arm power is desirable. A number of efficient interferometer designs have been proposed in the prior art (see for example, U.S. Pat. Nos. 7,388,672, 7,126,693, 7,145,661, 7,102,756, 6,657,727, 7,280,221, 6,501,551, 549,570, all of which are hereby incorporated by reference).
Some of the limitations that are associated with the prior-art interferometer designs are 1) lossy fiber coupling at the source which limits the utility of low power sources; 2) non-reciprocal beamsplitting ratios are implemented using faraday circulators, which are currently not available at low cost at ophthalmic wavelengths of 840 and 1050 nm; 3) incompatibility with dual balanced detection, typically required for swept source OCT; 4) fixed power ratios in the sample and reference arm, or lossy attenuating elements are required in the reference arm to set optimal power; and 5) need of polarization elements/optics in the reference and/or detection arms to select from input polarization states the components of the sample and reference which have the same polarization states.
Here, we propose some improved interferometer designs over the prior-art designs for use with OCT systems. The improved designs are cost effective (by involving minimal or low cost optics), improve the overall optical efficiency of light transmission for maximum signal detection, and are highly compatible with dual balanced detection.